spi: spi-qpic-snand: document the limited bit error reporting capability

The QPIC hardware is not capable of reporting the exact number of the
corrected bit errors, it only reports the number of the corrected bytes.

Document this behaviour in the code, and also issue a warning message
to inform the user about it.

No functional changes.

Signed-off-by: Gabor Juhos <j4g8y7@gmail.com>
Link: https://patch.msgid.link/20250527-qpic-snand-limited-biterr-caps-v1-1-61f7cf87be1e@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>